Na+K+Cl− Co-Transporters
In absorptive and secretory epithelia, transcellular ion transport depends on specific plasma membrane proteins for mediating ion entry into and exit from cells. In basolateral membrane of almost all epithelia (with exception of choroidal plexus), sodium exit and potassium entrance occur through Na+K+-ATPase, generating electrochemical gradients that constitute a driving force for Na+ influx and K+ efflux. Transport of these ions following their gradients can be accomplished by specific ion channels, allowing membrane passage of ions alone or by transporters in which Na+ or K+ transport is accompanied by other ions or solutes by means of several different solute transporters. These membrane proteins are known as secondary transporters because ion or molecule translocation is not dependent on ATP hydrolysis but rather on gradients generated by primary transporters. A secondary transport mechanism that is very active in transcellular ion transport in epithelial cells is one in which cations (Na+ or K+) are coupled with chloride, with a stoichiometry of 1:1; therefore, ion translocation produces no change in transmembrane potential. For this reason, these transporters are known as electroneutral cation-Cl− coupled cotransporters. In addition to being heavily implicated in ion absorptive and secretory mechanisms, electroneutral cation-Cl− coupled cotransporters play a key role in maintenance and regulation of cell volume in both epithelial and nonepithelial cells. Because Na+ influx and K+ efflux by electroneutral cotransporters are rapidly corrected by Na+K+-ATPase, the net effect of its activity is Cl− movement inside or outside cells. This is known to be accompanied by changes in cell volume. Finally, a variety of new physiological roles for electroneutral cotransporters are emerging (e.g., regulation of intraneuronal Cl− concentration and thus modulation of neurotransmission.) Gamba (2005) “Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters.” Physiol. Rev. 85: 423-493.
Four groups of electroneutral cotransporter systems (also known as “symporters”) have been functionally identified based on cation(s) coupled with chloride, stoichiometry of transport process, and sensitivity to inhibitors. These systems include: (1) the benzothiadiazine (or thiazide)-sensitive Na+Cl− cotransporter, (2) the sulfamoylbenzoic (or bumetanide) sensitive Na+K+2Cl− cotransporters; (3) the sulfamoylbenzoic (or bumetanide) sensitive Na+Cl− cotransporters; and (4) the dihydroindenyloxyalkanoic acid (DIOA)-sensitive K+Cl− cotransporter. There is some overlap in sensitivity to inhibitors in the last two groups because Na+K+2Cl− and K+Cl− cotransporters can be inhibited by high concentration of DIOA or loop diuretics, respectively; however, affinity for inhibitor and the cation coupled with chloride clearly differentiate between both groups of transporters. Physiological evidence for these transport mechanisms became available at the beginning of the 1980s, and a remarkable amount of information was generated in the following years by characterizing these transport systems in many different cells and experimental conditions. Gamba (2005) “Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters.” Physiol. Rev. 85: 423-493.
One isoform of the Na+K+Cl− cotransporter (NKCC) NKCC1 is widely distributed throughout the body. NKCC1 transports sodium, potassium, and chloride into the cell. NKCC1 is also found throughout the nervous system where it is expressed on astrocytes, oligodendrocytes, and Schwann cells. Lenart, et al. (2004) The Journal of Neuroscience 24(43): 9585-9597. Another isoform, NKCC2 is found primarily in the kidney, where it serves to extract sodium, potassium, and chloride from the urine. Haas (1994) “The Na—K—Cl cotransporters.” Am J Physiol Cell Physiol 267: C869-C885. Bumetanide, furosemide, piretanide, azosemide, and torsemide are loop diuretics that have a potent diuretic effect. Loop diuretics are antagonists of the Na+K+Cl− cotransporter (e.g., NKCC2) in the thick ascending limb of the loop of Henle and act to inhibit sodium and chloride reabsorption by competing for the Cl− binding site. See also Russell (January 2000) “Sodium-Potassium-Chloride Cotransport.” Physioglocal Reviews 80(1): 211-275.
The regulation of Cl− transport into and out of cells plays a critical role in the maintenance of intracellular volume and the excitability of GABA responsive neurons regulated by at least two ion cotransporters: Cl− influx is mediated by the NKCC1 which mediates the Cl− influx and KCC1 or KCC2 which mediates the Cl− efflux. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788. The maintenance of intra- and extracellular electrolyte homeostasis is required for a wide range of essential physiologic processes, including general functions (e.g., maintenance of proper cell volume), specialized cell functions (e.g., control of neuronal excitability), and global functions (e.g., regulation of blood pressure.) This homeostasis is achieved via the regulated movement of Na+, K+, and Cl− across cell membranes by ion channels, cotransporters, exchangers, and pumps that execute transmembrane electrolyte flux. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788.
The predominant mechanism by which intracellular volume is maintained in cells in response to changes in extracellular tonicity is the raising or lowering of intracellular Cl− concentration ([Cl−]i), thereby minimizing transmembrane water flux. [Cl−]i is modulated by altering the balance between Cl− entry and exit. The major mediator of Cl− entry is NKCC1 and Cl− exit is largely mediated by KCC1. These cotransporters are both regulated by extracellular tonicity: hypertonicity activates NKCC1 and inhibits KCC1, whereas hypotonicity has the opposite effect. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788.
An analogous system plays a key role in the control of neuronal excitability. In the adult brain, GABA is the major inhibitory neurotransmitter. If [Cl−]i is below its equilibrium potential, Cl− enters the cell, resulting in hyperpolarization and inhibition. If [Cl−]i is above its equilibrium potential, GABA induces Cl− efflux, depolarization, and neuronal excitation. The importance of [Cl−]i regulation has been recognized with the discovery that GABA neurotransmission is not uniformly inhibitory; it is predominantly excitatory in the neonatal period. Similarly, neurons of the suprachiasmatic nucleus show circadian variation in their response to GABA, demonstrating the ability to dynamically regulate [Cl−]i. Finally, GABA neurotransmission in the peripheral nervous system is predominantly excitatory. Variation in [Cl−]i in these neurons is determined by mechanisms highly similar to those governing cell volume. Cl− influx largely occurs via NKCC1, whereas Cl− efflux is mediated via the neuronal-specific K—Cl cotransporter KCC2. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788.
The mediators of transcellular Cl− cotransport (Na—Cl cotransporter, NKCC1, NKCC2, KCC1, and KCC2) are all related members of the SLC12A family of cation/Cl− cotransporters; each takes advantage of inward Na+ or outward K+ gradients to move Cl− into or out of cells, respectively. The importance of this family of transporters is underscored by their use as pharmacologic targets (thiazide diuretics act at NCC, and loop diuretics act at NKCC2), and that their mutation results in diverse diseases. Disruption of NKCC1 in mouse leads to hearing loss, altered pain perception, neuronal excitability, and altered blood pressure. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788.
Major advances have been made in the past decade in molecular identification and characterization of solute carriers. As of 2005, the Human Genome Organization (HUGO) Nomenclature Committee Database recognizes 43 solute carries (SLC) families, which include a total of 298 transporter genes encoding for uniporters (passive transporters), cotransporters (coupled transporters), antiporters (exchangers), vesicular transporters, and mitochondrial transporters. This amount of solute carrier genes represents ˜1% of the total pool of genes that have been calculated to compose human genome. Gamba (2005) “Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters.” Physiol. Rev. 85: 423-493.
GABA Receptors
Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system (CNS) where approximately 30% of all synapses use GABA as a transmitter. There are three classes of GABA receptors: GABAA (ligand-gated ion channel), GABAB (G protein-coupled receptor), and GABAC (ligand-gated ion channel). Chloride flux into the cell results from the activation of GABAA receptors by the binding of GABA molecules, hyperpolarizing the resting membrane potential and decreasing the chances of the post-synaptic neuron propagating an action potential. GABAA receptors are pentameric and approximately 19 GABA receptor subunits have been cloned from mammals (6α, 3β, 3γ, 1 δ, 1ε, 1θ, 1π, and 3ρ subunits). The heterogeneity of GABA subunits is further increased by alternate splicing (e.g., γ2 short and γ2 long are the two major splice variants of the γ2). In general, a functional GABAA receptor requires 2α subunits, 2β subunits and a third “regulatory” subunit (usually γ or δ). WO 2009/100040. The specific subunit combination determines the pharmacological and ligand binding properties of the GABAA receptor. The most abundant subunit combination found in the CNS is α1β2γ2. This subtype represents approximately 40% of GABAA receptors in the brain and it is expressed throughout the CNS and is located on post-synaptic cells. WO 2007/002359.
The GABAA receptors are the targets of a wide range of therapeutic and clinically relevant compounds including benzodiazepines, barbiturates, neurosteroids, ethanol, certain intravenous anesthetics, and subtype specific modulators (e.g., Zolpidem.) These compounds serve as anxiolytics, sedative/hypnotics, anti-epileptic drugs (AED), and memory enhancers. Many of these therapeutics show efficacy but cause side effects due to unwanted effects at α1 and/or α2 GABAA variants or due to low therapeutic index. For example, benzodiazepines such as diazepam (VALIUM) are excellent anxiolytics but cause unwanted sedative effects when used clinically. WO 2007/002359.
At a cellular level, GABAA receptors are expressed both at both post-synaptic and extra-synaptic sites (parasynaptic) where they respond to large changes in GABA concentration caused by release of the neurotransmitter into the synaptic space, and extra-synaptically where the receptors respond to lower concentrations of GABA that “leak” from synaptic junctions. The post-synaptic receptors respond to acute changes in neuronal firing whereas the extrasynaptic receptors are responsible for maintaining overall tone of neuronal networks. WO 2009/100040. Tonic inhibition is generated by the persistent activation of extrasynapatic (perisynaptic) GABAA receptors and regulates the excitability of individual neurons and neural networks. Jia, et al. et al. (2008) The Journal of Pharmacology and Experimental Therapeutics 326(2): 475482.
Presynaptic GABAA receptors situated at extrasynaptic sites may comprise α4βδ and α6βδ isoforms. The extrasynaptic α4βδ and α6βδ GABAA receptor isoforms show marked sensitivity to GABA, alcohol, and anesthetics, suggesting that receptors may present a critical site for regulating synaptic function in the developing brain in both physiological and pathological situations. Xiao, et al. et al. (2007) “Presynaptic GABAA receptors facilitate GABAergic transmission to dopaminergic neurons in the ventral tegmental area of young rats.” J Physiol. 580(Pt.3):731-43. For example, temporal lobe epilepsy (TLE), Parkinson's disease (PD) and Huntington's disease (HD) are neurodegenerative disorders that involve disruptions in GABA signaling. GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). TLE seizures reflect excess excitation, which may result from local inhibitory circuit dysfunction. PD devastates the input to striatal GABAergic neurones and HD destroys striatal GABAergic neurones. Controlling GABA delivery to specific brain areas should benefit each of these diseases. Directing GABA synthesis, degradation, transport or receptors can control GABA signaling. New drugs targeting GABA synthesis, release, and binding may be used for improved therapeutics treatments for epilepsy and both Parkinson's and Huntington's disease. Kleppner and Tobin (2001) “GABA signaling: therapeutic targets for epilepsy, Parkinson's disease and Huntington's disease.” Expert Opin Ther Targets. 5(2):219-39. See also Shumate, et al. et al. (1998) Epilepsy Research (32): 114-128; Fritschy (2008) Frontiers in Molecular Neuroscience 1(5): 1-5; Roberts, et al. et al. (2006) The Journal of Biological Chemistry 281(40): 29431-29435; and Roberts, et al. et al. PNAS 102(33): 11894-11899.
Addictive Disorders
Addictive and/or compulsive disorders, such as eating disorders (including obesity), addiction/physical dependence to stimulants, narcotics (e.g., cocaine, heroin) sedatives/hypnotics, and opioids including alcoholism and smoking are major public health problem that impact society on multiple levels. It has been estimated that substance abuse alone costs the United States more than $484 billion per year.
The alcohol-sensitive α4βδ GABAA receptor has also been postulated to be involved in alcohol addiction (alcoholism). For example, reduced expression of GABAA receptors comprising an α4 subunit in the nucleus accumbens (NAc) decreased the free consumption and preference for alcohol in rats. Further, the nucleus accumbens contributes to the rewarding and reinforcing effects of drugs including alcohol suggesting that the GABAA receptor, specifically the α4βδ isoform, in the NAc is an important mediator of alcohol self-administration. Rewal, et al. et al. (Jan. 14, 2009) The Journal of Neuroscience 29(2): 543-549.
Although most GABAA receptor subunit combinations can be activated by high (anesthetic) alcohol concentrations, so far only very specific GABAA receptor subunit combinations (containing the δ as well as the β3 subunit) exhibit dose-dependencies that mirror blood alcohol levels associated with mild to moderate intoxication in humans. GABAA receptors containing the δ subunit are located either outside or in the perimeter of synapses, but not in the sub-synaptic membrane. WO 2007/002359.
Current strategies for the treatment of additive disorders include psychological counseling and support, use of therapeutic agents, or a combination of both. A variety of agents known to affect the central nervous system have been used in various contexts to treat a number of indications related directly or indirectly to addictive behaviors but a great need remains for improved addictive disorder therapeutics. GABAA specific agents that have action at the nucleaus accumbens might be effective therapies for addictive behaviors.
Alzheimer's Disease
Alzheimer's disease (AD) is an age-related, non-reversible brain disorder that develops over a period of years. Initially, people experience memory loss and confusion, which may be mistaken for the kinds of memory changes that are sometimes associated with normal aging. However, the symptoms of AD gradually lead to behavior and personality changes, a decline in cognitive abilities such as decision-making and language skills, and problems recognizing family and friends. AD ultimately leads to a severe loss of mental function. AD is the most common cause of dementia among people age 65 and older. NINDS Alzheimer's Disease Information Page (Sep. 23, 2009).
The three major hallmarks in the brain that are associated with the disease processes of AD include amyloid plaques and neurofibrillary tangles. Amyloid plaques comprise fragments of β-amyloid peptide mixed with a collection of additional proteins, and remnants of neurons. Neurofibrillary tangles (NFTs) are found inside neurons and comprise tau protein. As neurons die throughout the brain, the affected regions begin to atrophy. By the final stage of AD, damage is widespread and brain tissue has shrunk significantly. NINDS Alzheimer's Disease Information Page (Sep. 23, 2009).
Currently there are no medicines that can slow the progression of AD. However, four FDA-approved medications are used to treat AD symptoms. Unfortunately these medications will not stop or reverse AD, and they appear to help individuals for only a few months to a few years. Donepezil (ARICEPT), rivastigmine (EXELON), and galantamine (REMINYL) are prescribed to treat mild to moderate AD symptoms. Donepezil was recently approved to treat severe AD as well. The newest AD medication is memantine (NAMENDA), which is prescribed to treat moderate to severe AD symptoms. NINDS Alzheimer's Disease Information Page (Sep. 23, 2009). Treatment of a transgenic mouse model of Alzheimer's Disease with picrotoxin, a GABAA antagonist showed improved cognitive functions in the mice. Yoshiike, et al. et al. (Aug. 21, 2008) “GABAA receptor-mediated acceleration of aging-associated memory decline in APP/PSI mice and its pharmacological treatment by picrotoxin.” PLoS One. 3(8):e3029. Additionally, the expression of NKCC1 has been found to be elevated in Alzheimer's Disease patients. Johanson, et al. (2004) Cerebrospinal Fluid Research 1:3. Therefore novel therapies based on the regulation of GABAA receptor activity may relieve the symptoms of AD.
Anxiety Disorders
Anxiety disorders are classified into several subtypes: anxiety, acute anxiety, panic disorder, social anxiety disorder, obsessive compulsive disorder (OCD), post-traumatic stress disorder (PTSD), generalized anxiety disorder, and specific phobia. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 4th edition (1994).
As a group, the anxiety disorders have the highest prevalence in the U.S. of all psychiatric disorders. Kessler, et al. et al. (1994) “Lifetime and 12-month prevalence of DSM-III-R psychiatric disorders in the United States: Results from the National Comorbidity Survey.” Arch Gen Psychiatry 51:8-19. Anxiety disorders afflict 15.7 million people in the United States each year, and 30 million people in the United States at some point in their lives. Lepine (2002) “The Epidemiology of Anxiety Disorders: Prevalence and Societal Costs.” J. Clin. Psychiatry. 63: Suppl 14:4-8.
Several animal models have been developed which are recognized in the art as being predictive of anxiolytic activity. These include the fear-potentiated startle model, described by Davis in Psychopharmacology 62:1; 1979, Behav. Neurosci. 100:814; 1986 and TiPS, January 1992 Vol. 13, 35-41, the elevated plus model described by Lister in Psychopharmacol. 92:180-185; 1987, and the well-known punished—responding (conflict) model, described, for example, in “Psychopharmacology of Anxiolytics and Antidepressants”, edited by S. E. File, pp. 131-153, Raven Press, New York, 1991.
Anxiety disorders are generally treated with drugs and psychotherapy. The most commonly prescribed drugs for all anxiety types are the benzodiazepines, other psychoactive drugs such as selective serotonin reuptake inhibitors (SSRI) are also used. However, while these drugs show efficacy, both benzodiazepines and SSRIs are also show adverse effects during treatment. Denys and de Geus (August 2005) “Predictors of Pharmacotherapy Response in Anxiety Disorders.” Curr Psychiatry Rep. 7(4): 252-7. For example, numerous side effects are associated with long-term use of SSRIs, such as sexual dysfunction and weight gain. Hirschfeld (2003) “Long-term Side Effects of SSRIs: Sexual Dysfunction and Weight Gain.” J. Clin. Psychiatry. 64: Suppl 18: 20-4. Further, existing drugs targeting postsynaptic type GABAA receptors produce undesirable results because they indiscriminately target most of the GABAA receptors in the brain. WO 2007/136838. In view of nonresponders and deleterious side effects, a great need exists for improved anxiety therapeutics.
Ascites
Ascites is excess fluid in the space between the tissues lining the abdomen and abdominal organs (the peritoneal cavity) typically caused by liver disease. Disorders that may be associated with ascites include: cirrhosis, hepatitis, portal vein thrombosis, constrictive pericarditis, congestive heart failure, liver cancer, ovarian cancer, protein-losing enteropathy, nephrotic syndrome, and pancreatitis. Some agents are available for the treatment of ascites, for example, furosemide but a great need remains for improved ascites therapeutics. See Shiozaki, et al. (2006) J. Physiol. Sci. 56(6): 401-406.
Autism
Autism spectrum disorder (ASD) is a range of complex neurodevelopment disorders, characterized by social impairments, communication difficulties, and restricted, repetitive, and stereotyped patterns of behavior. Autistic disorder, sometimes called autism or classical ASD, is the most severe form of ASD, while other conditions along the spectrum include a milder form known as Asperger syndrome, the rare condition called Rett syndrome, and childhood disintegrative disorder and pervasive developmental disorder not otherwise specified (usually referred to as PDD-NOS). Although ASD varies significantly in character and severity, it occurs in all ethnic and socioeconomic groups and affects every age group. Experts estimate that three to six people out of every 1,000 may develop ASD. Males are four times more likely to have ASD than females. NINDS Autism Fact Sheet (September 2009). Furthermore, various GABA A subunit types, such as α3 variant sequences and α4 isoforms have been linked to autism. WO 2009/100040. There is no cure for ASD, thus there exists a great need for therapeutics to treat autism.
Multiple lines of evidence, including genetic and imaging studies, suggest that the anterior cingulate cortex and gamma-amino-butyric acid (GABA) system may be affected in autism. Compared to controls, the autistic patients show a significant decrease in the mean density of GABAA receptors in the supragranular (46.8%) and infragranular (20.2%) layers of the anterior cingulate cortex (ACC) and in the density of benzodiazepine binding sites in the supragranular (28.9%) and infragranular (16.4%) lamina. In addition, a trend for a decrease in the density of benzodiazepine sites was found in the infragranular layers (17.19%) in the autism group. These findings suggest that in the autistic group this downregulation of both benzodiazepine sites and GABAA receptors in the ACC may be the result of increased GABA innervation and/or release disturbing the delicate excitation/inhibition balance of principal neurons as well as their output to key limbic cortical targets. These disturbances may underlie the core alterations in socio-emotional behaviors in autism. Oblak, et al. et al. (August 2009) “Decreased GABAA receptors and benzodiazepine binding sites in the anterior cingulate cortex in autism.” Autism Res. 2009 August; 2(4):205-19. Therefore, therapeutics that target the GABAA receptor may be useful in treating autism.
Bipolar Disorder
Bipolar disorder, also known as manic-depressive illness, is a brain disorder that causes unusual shifts in a person's mood, energy, and ability to function. They can result in damaged relationships, poor job or school performance, and even suicide. About 5.7 million American adults or about 2.6 percent of the population age 18 and older have bipolar disorder in any given year. Bipolar disorder typically develops in late adolescence or early adulthood. However, some people have their first symptoms during childhood, and some develop them late in life. It is often not recognized as an illness, and people may suffer for years before it is properly diagnosed and treated. National Institute of Mental Health “Bipolar Disorder” (2008) Complete Publication.
Bipolar disorder causes dramatic mood swings—from overly “high” and/or irritable to sad and hopeless, and then back again, often with periods of normal mood in between. Severe changes in energy and behavior go along with these changes in mood. The periods of highs and lows are called episodes of mania and depression. National Institute of Mental Health “Bipolar Disorder” (2008) Complete Publication.
Signs and symptoms of mania (or a manic episode) include: increased energy, activity, and restlessness; excessively “high,” overly good, euphoric mood; extreme irritability; racing thoughts and talking very fast, jumping from one idea to another, distractibility, difficulty concentrating; little sleep needed; unrealistic beliefs in one's abilities and powers; poor judgment; spending sprees; a lasting period of behavior that is different from usual; increased sexual drive; drug abuse, particularly cocaine, alcohol, and sleeping medications; provocative, intrusive, or aggressive behavior; and/or denial that anything is wrong. A manic episode is diagnosed if elevated mood occurs with three or more of the other symptoms most of the day, nearly every day, for 1 week or longer. National Institute of Mental Health “Bipolar Disorder” (2008) Complete Publication.
Signs and symptoms of depression (or a depressive episode) include: lasting sad, anxious, or empty mood; feelings of hopelessness or pessimism; feelings of guilt, worthlessness, or helplessness; loss of interest or pleasure in activities once enjoyed, including sex; decreased energy, a feeling of fatigue or of being “slowed down”; difficulty concentrating, remembering, making decisions; restlessness or irritability; sleeping too much, or cannot sleep; change in appetite and/or unintended weight loss or gain; chronic pain or other persistent bodily-symptoms that are not caused by physical illness or injury; and/or thoughts of death or suicide, or suicide attempts. A depressive episode is diagnosed if five or more of these symptoms last most of the day, nearly every day, for a period of 2 weeks or longer; National Institute of Mental Health “Bipolar Disorder” (2008) Complete Publication.
A mild to moderate level of mania is called hypomania. Hypomania may feel good to the person who experiences it and may even be associated with good functioning and enhanced productivity. Thus even when family and friends learn to recognize the mood swings as possible bipolar disorder, the person may deny that anything is wrong. Without proper treatment, however, hypomania can become severe mania in some people or can switch into depression.
In some people, however, symptoms of mania and depression may occur together in what is called a mixed bipolar state. Symptoms of a mixed state often include agitation, trouble sleeping, significant change in appetite, psychosis, and suicidal thinking. A person may have a very sad, hopeless mood while at the same time feeling extremely energized.
The classic form of the illness, which involves recurrent episodes of mania and depression, is called bipolar I disorder. Some people, however, never develop severe mania but instead experience milder episodes of hypomania that alternate with depression; this form of the illness is called bipolar II disorder. When four or more episodes of illness occur within a 12-month period, a person is said to have rapid-cycling bipolar disorder. Some people experience multiple episodes within a single week, or even within a single day. Rapid cycling tends to develop later in the course of illness and is more common among women than among men.
Medications known as “mood stabilizers” usually are prescribed to help control bipolar disorder (e.g. lithium or valproic acid-DEPAKOTE/VALPROATE). In addition to medication, psychosocial treatments—including certain forms of psychotherapy, are often used to treat bipolar disorders. Depending on the medication, side effects include weight gain, nausea, tremor, reduced sexual drive or performance, anxiety, hair loss, movement problems, or dry mouth. Lithium treatment can cause low thyroid levels, resulting in the need for thyroid supplementation. Additionally, Valproate® may lead to adverse hormone changes in teenage girls and polycystic ovary syndrome in women who began taking the medication before age 20. Further, women suffering bipolar disorder who wish to conceive, or who become pregnant, face special challenges due to the possible harmful effects of existing mood stabilizing medications on the developing fetus and the nursing infant. National Institute of Mental Health “Bipolar Disorder” (2008) Complete Publication. Improved bipolar disorder therapeutics may be developed that act to increase GABA activity.
Postmortem and genetic studies have linked neuropsychiatric disorders including schizophrenia and bipolar disorder with GABAergic neurotransmission and various specific GABAA receptor subunits. Further, GABAA receptor-associated proteins involved in GABAA receptor trafficking, targeting, clustering, and anchoring that often carry out these functions in a subtype-specific manner. Charych, et al. (2009) “GABAA receptors and their associated proteins: implications in the etiology and treatment of schizophrenia and related disorders. Neuropharmacology. 57(5-6): 481-95. Therefore, GABAA receptor specific therapeutics that improve inhibition may be beneficial because bipolar disease is a state of alterations of abnormal inhibition/excitation without adequate inhibition.
Cognition, Learning, and Memory
The cognitive abilities of mammals are thought to be dependent on cortical processing. It has generally been accepted that the most relevant parameters for describing and understanding cortical function are the spatio-temporal patterns of activity. In particular, long-term potentiation and long-term depression have been implicated in memory and learning and may play a role in cognition. Oscillatory and synchronized activities in the brains of mammals have been correlated with distinct behavioral states.
Synchronization of spontaneous neuronal firing activity is thought to be an important feature of a number of normal and pathophysiological processes in the central nervous system. Examples include synchronized oscillations of population activity such as gamma rhythms in the neocortex, which are thought to be involved in cognition (Singer and Gray (1995) Annu. Rev. Neurosci. 18: 855-86), and theta rhythm in hippocampus, which is thought to play roles in spatial memory and in the induction of synaptic plasticity (Heurta and Lisman (1995) Neuron. 15: 1053-63; Heurta and Lisman (1996) J. Neurophysiol. 75: 877-84; O'Keefe (1993) Curr. Opin. Neurobiol. 3: 917-24). To date, most research on the processes underlying the generation and maintenance of spontaneous synchronized activity has focused on synaptic mechanisms. However, there is evidence that nonsynaptic mechanisms may also play important roles in the modulation of synchronization in normal and pathological activities in the central nervous system.
Depression
Depression is a common but serious illness, the most common are major depressive disorder and dysthymic disorder. Major depressive disorder, also called major depression, is characterized by a combination of symptoms that interfere with a person's ability to work, sleep, study, eat, and enjoy once-pleasurable activities. Major depression is disabling and prevents a person from functioning normally. An episode of major depression may occur only once in a person's lifetime, but more often, it recurs throughout a person's life. National Institute of Mental Health “Depression” (2008) Complete Publication.
The forms of depression include:
Dysthymic disorder, also called dysthymia, is characterized by long-term (two years or longer) but less severe symptoms that may not disable a person but can prevent one from functioning normally or feeling well. People with dysthymia may also experience one or more episodes of major depression during their lifetimes.
Psychotic depression, which occurs when a severe depressive illness is accompanied by some form of psychosis, such as a break with reality, hallucinations, and delusions. Postpartum-depression, which is diagnosed if a new mother develops a major depressive episode within one month after delivery. It is estimated that 10 to 15 percent of women experience postpartum depression after giving birth.
Seasonal affective disorder (SAD), which is characterized by the onset of a depressive illness during the winter months, when there is less natural sunlight. The depression generally lifts during spring and summer. SAD may be effectively treated with light therapy, but nearly half of those with SAD do not respond to light therapy alone. Antidepressant medication and psychotherapy can reduce SAD symptoms, either alone or in combination with light therapy. National Institute of Mental Health “Depression” (2008) Complete Publication.
Depression can be treated with a number of methods. The most common treatments are medication and psychotherapy. Antidepressants work to normalize neurotransmitters, notably serotonin, norepinephrine, and dopamine. The newest and among the most popular types of antidepressant medications are called selective serotonin reuptake inhibitors (SSRIs). SSRIs include fluoxetine (Prozac®), citalopram (Celexa®), sertraline (Zoloft®), and several others. Serotonin and norepinephrine reuptake inhibitors (SNRIs) are similar to SSRIs and include venlafaxine (Effexor®) and duloxetine (Cymbalta®). SSRIs and SNRIs are more popular than the older classes of antidepressants, such as tricyclics-named for their chemical structure- and monoamine oxidase inhibitors (MAOIs) because they tend to have fewer side effects. However, medications affect everyone differently-no one-size-fits-all approach to medication exists. National Institute of Mental Health “Depression” (2008) Complete Publication.
For all classes of antidepressants, patients can experience side effects. Antidepressants may cause mild and often temporary side effects in some people, but they are usually not long-term. The most common side effects associated with SSRIs and SNRIs include: headache, nausea, insomnia, nervousness, agitation, and sexual problems. National Institute of Mental Health “Depression” (2008) Complete Publication. Tricyclic antidepressants also can cause side effects including: dry mouth, constipation, bladder problems, sexual problems, blurred vision, and daytime drowsiness. Additionally, patients taking MAOIs must adhere to significant food and medicinal restrictions to avoid potentially serious interactions. They must avoid certain foods that contain high levels of the chemical tyramine, which is found in many cheeses, wines and pickles, and some medications including decongestants. MAOIs interact with tyramine in such a way that may cause a sharp increase in blood pressure, which could lead to a stroke. National Institute of Mental Health “Depression” (2008) Complete Publication.
GABA is involved in both clinical depression and in animal models of depression. Kram, et al. (October 2000) “Effects of learned helplessness on brain GABA receptors.” Neuroscience Research 38(2): 193-198. Therefore improved depression therapeutics based on the GABAergic system may provide better medication.
Epilepsy
Epilepsy is characterized by abnormal discharges of cerebral neurons and is typically manifested as various types of seizures. Epileptiform activity is identified with spontaneously occurring synchronized discharges of neuronal populations that can be measured using electrophysiological techniques. Epilepsy is one of the most common neurological disorders, affecting about 1% of the population. There are various forms of epilepsy, including idiopathic, symptomatic, and cryptogenic. Genetic predisposition is thought to be the predominant etiologic factor in idiopathic epilepsy. Symptomatic epilepsy usually develops as a result of a structural abnormality in the brain.
Status epilepticus is a particularly severe form of seizure, which is manifested as multiple seizures that persist for a significant length of time, or serial seizures without any recovery of consciousness between seizures. The overall mortality rate among adults with status epilepticus is approximately 20 percent. Patients who have a first episode are at substantial risk for future episodes and for the development of chronic epilepsy. The frequency of status epilepticus in the United States is approximately 150,000 cases per year, with approximately 55,000 deaths being associated with status epilepticus annually. Sirven and Waterhouse (2003) “Management of Status Epilepticus” American Family Physician 68: 469-476. Acute processes that are associated with status epilepticus include intractable epilepsy, metabolic disturbances (e.g., electrolyte abnormalities, renal failure, and sepsis), central nervous system infection (meningitis or encephalitis), stroke, degenerative diseases, head trauma, drug toxicity, and hypoxia. The fundamental pathophysiology of status epilepticus involves a failure of mechanisms that normally abort an isolated seizure. This failure can arise from abnormally persistent, excessive excitation or ineffective recruitment of inhibition. Studies have shown that excessive activation of excitatory amino acid receptors can cause prolonged seizures and suggest that excitatory amino acids may play a causative role. Status epilepticus can also be caused by penicillin and related compounds that antagonize the effects of γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain.
One early diagnostic procedure for epilepsy involved the oral administration of large quantities of water together with injections of vasopressin to prevent the accompanying diuresis. This procedure was found to induce seizures in epileptic patients, but rarely in non-epileptic individuals. Garland, et al. (1943) Lancet 2: 566. Status epilepticus can be blocked in kainic acid-treated rats by intravenous injection of mannitol. Baran, et al. (1987) Neuroscience 21: 679. This effect is similar to that achieved by intravenous injection of urea in human patients. Carter (1962) Epilepsia 3: 198. The treatment in each of these cases increases the osmolarity of the blood and extracellular fluid, resulting in water efflux from the cells and an increase in extracellular space in the brain. Acetazolamide (ACZ), a diuretic with a different mechanism of action (inhibition of carbonic anhydrase), has been studied experimentally as an anticonvulsant (White, et al. (1986) Advance Neurol. 44: 695; Guillaume, at al. (1991) Epilepsia 32: 10) and used clinically on a limited basis (Tanimukai, et al. (1965) Biochem. Pharm. 14: 961; Forsythe, et al. (1981) Develop. Med. Child Neurol. 23: 761). Although its mechanism of anticonvulsant action has not been determined, ACZ does have a clear effect on the cerebral extracellular space.
Traditional anti-epileptic drugs exert their principal effect through one of three mechanisms: (a) inhibition of repetitive, high-frequency neuronal firing by blocking voltage-dependent sodium channels; (b) potentiation of γ-aminobutyric acid (gamma-aminobutyric acid, GABA)-mediated postsynaptic inhibition; and (c) blockade of T-type calcium channels.
Many current anti-epileptic drug therapies exert their pharmacological effects on all brain cells, regardless of their involvement in seizure activity. Common side effects are over-sedation, dizziness, loss of memory and liver damage. Furthermore, 20-30% of epilepsy patients are refractory to current therapy. Therefore there is a great need for improved epilepsy therapeutics to reduce both morbidity and mortality.
Glaucoma
Glaucoma is a group of diseases that can damage the eye's optic nerve and result in vision loss and blindness. Glaucoma occurs when the normal fluid pressure inside the eyes slowly rises. Open-angle glaucoma is the most common form. Other types of glaucoma include: (1) low-tension or normal-tension glaucoma; (2) angle-closure glaucoma; (3) congenital glaucoma; (4) secondary glaucomas; and (5) pigmentary glaucoma including neovascular glaucoma. Everyone is at risk for glaucoma but some populations are at higher risk than others including African Americans over age 40, everyone over age 60, and people with a family history of glaucoma. National Eye Institute Glaucoma Fact Sheet (2008).
Glaucoma is usually detected through a comprehensive eye exam that includes: (a) visual acuity test; (b) visual field test; (c) dilated eye exam; (d) tonometry; and (e) pachymetry. Current glaucoma treatments include medicines, laser trabeculoplasty, conventional surgery, or a combination of any of these, however there is a great need for improved glaucoma therapeutics. National Eye Institute Glaucoma Fact Sheet (2008).
Huntington's Disease
Huntington's disease (HD) results from neuronal degeneration leading to uncontrolled movements, loss of intellectual faculties, and emotional disturbance. HD is an autosomal dominant disease caused by a CAG expansion in the Htt gene that leads to a poly-glutamine expansion in the disease protein huntingtin. GABAergic interneurons are particularly sensitive to the accumulation of mutant huntingtin and die early in the development of HD. Some early symptoms of HD are mood swings, depression, irritability or trouble driving, learning new things, remembering a fact, or making a decision. As the disease progresses, concentration on intellectual tasks becomes increasingly difficult, and the patient may have difficulty feeding himself or herself and swallowing. The rate of disease progression and the age of onset vary from person to person. A genetic test, coupled with a complete medical history and neurological and laboratory tests, helps physicians diagnose HD. Presymptomic testing is available for individuals who are at risk for carrying the HD gene. In 1 to 3 percent of individuals with HD, no family history of HD can be found. NINDS Publication “Huntington's Disease: Hope Through Research” (2009).
Physicians prescribe a number of medications to help control emotional and movement problems associated with HD. In August 2008 the U.S. Food and Drug Administration approved tetrabenazine to treat Huntington's chorea (the involuntary writhing movements), making it the first drug approved for use in the United States to treat the disease. However, drugs used to treat the symptoms of HD have side effects such as fatigue, restlessness, or hyperexcitability. NINDS Publication “Huntington's Disease: Hope Through Research” (2009).
Huntington's disease (HD) is a neurodegenerative disorder that involves disruptions in GABA signaling. GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). HD destroys striatal GABAergic neurons. Directing GABA synthesis, degradation, transport, or receptors can control GABA signaling and so drugs that target these aspects of GABA metabolism may be used for improved therapeutic treatments, for Huntington's disease. Kleppner and Tobin (2001) “GABA signaling: therapeutic targets for epilepsy, Parkinson's disease and Huntington's disease.” Expert Opin Ther Targets. 5(2):219-39.
Insomnia
Insomnia is a symptom of any of several sleep disorders, characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. NHLBI Diseases and Conditions Index [Insomnia](October 2009).
Although there are several different degrees of insomnia, three types of insomnia have been clearly identified: transient, acute, and chronic. Transient insomnia lasts from days to weeks. It can be caused by another disorder, by changes in the sleep environment, by the timing of sleep, severe depression, or by stress. Its consequences—sleepiness and impaired psychomotor performance—are similar to those of sleep deprivation. Acute insomnia is the inability to consistently sleep well for a period of between three weeks to six months. Chronic insomnia lasts for years at a time. It can be caused by another disorder, or it can be a primary disorder. Its effects can vary according to its causes. They might include sleepiness, muscular fatigue, hallucinations, and/or mental fatigue; but people with chronic insomnia often show increased alertness. NHLBI Diseases and Conditions Index [Insomnia] (October 2009).
Current insomnia drug therapies that target the GABAA receptor, hypnotics (e.g., benzodiazepines) may have undesirable side effects, therefore a great need exists for improved insomnia therapeutics with reduced side effects.
Ischemia
Ischemia is a restriction in blood supply, generally due to factors in the blood vessels, with resultant damage or dysfunction of tissue due to inadequate oxygenation and lack of nutrients of the tissue. Insufficient blood supply causes tissue to become hypoxic, or, if no oxygen is supplied at all, anoxic. In contrast with hypoxia, a more general term denoting a shortage of oxygen (usually a result of lack of oxygen in the air being breathed), ischemia is an absolute or relative shortage of the blood supply to an organ. This can cause necrosis (e.g., cell death). In aerobic tissues such as heart and brain, at body temperature necrosis due to ischemia usually takes about 3-4 hours before becoming irreversible. Later, more damage occurs due to the accumulation of metabolic wastes due to lack of adequate blood supply to the tissue. Complete cessation of oxygenation of such organs for more than 20 minutes typically results in irreversible damage.
Inhibition of NKCC1 activity with bumetanide and furosemide significantly reduces the infarct volume and cerebral edema following cerebral focal ischemia suggesting that NKCC1 antagonists may be useful in treating ischemia. Chen and Sun (2005) “The role of Na—K—Cl co-transporter in cerebral ischemia.” Neurol. Res. 27(3): 280-286. The typical treatment of ischemia involves “clot-buster” drugs (e.g., Alteplase®) usually given for stroke and heart attack within this time period. However, restoration of blood flow after a period of ischemia can actually be more damaging than the ischemia because reintroduction of oxygen causes a greater production of damaging free radicals, resulting in reperfusion injury. Necrosis is greatly accelerated in reperfusion injury and therefore a great need exists for improved ischemia therapeutics.
Migraine
Migraine headaches afflict 10-20% of the U.S. population, with an estimated loss of 64 million workdays annually. Migraine headache is characterized by pulsating head pain that is episodic, unilateral or bilateral, lasting from 4 to 72 hours and often associated with nausea, vomiting, and hypersensitivity to light and/or sound. When accompanied by premonitory symptoms, such as visual, sensory, speech or motor symptoms, the headache is referred to as “migraine with aura,” formerly known as classic migraine. When not accompanied by such symptoms, the headache is referred to as “migraine without aura,” formerly known as common migraine. Both types evidence a strong genetic component, and both are three times more common in women than men. The precise etiology of migraine has yet to be determined. It has been theorized that persons prone to migraine have a reduced threshold for neuronal excitability, possibly due to reduced activity of the inhibitory neurotransmitter γ-aminobutyric acid (GABA). GABA normally inhibits the activity of the neurotransmitters serotonin (5-HT) and glutamate, both of which appear to be involved in migraine attacks. The excitatory neurotransmitter glutamate is implicated in an electrical phenomenon called cortical spreading depression, which can initiate a migraine attack, while serotonin is implicated in vascular changes that occur as the migraine progresses.
It has been suggested that cortical spreading depression (CSD) underlies migraines including migraines with visual aura. It is also believed that CSD underlies migraine as part of the trigeminal pain circuit. CSD is characterized by a short burst of intense depolarization in the occipital cortex, followed by a wave of neuronal silence and diminished evoked potentials that advance anteriorly across the surface of the cerebral cortex. Enhanced excitability of the occipital-cortex neurons has been proposed as the basis for CSD. The visual cortex may have a lower threshold for excitability and therefore is most prone to CSD. It has been suggested that mitochondrial disorders, magnesium deficiency, and abnormality of presynaptic calcium channels may be responsible for neuronal hyperexcitability. Welch (1997) “Pathogenesis of Migraine.” Seminars in Neurobiol. 17: 4. During a spreading depression event, profound ionic perturbations occur, which include interstitial acidification, extracellular potassium accumulation, and redistribution of sodium and chloride ions to intracellular compartments. In addition, prolonged glial swelling occurs as a homeostatic response to altered ionic extracellular fluid composition, and interstitial neurotransmitter and fatty acid accumulation. Studies have shown that furosemide inhibits regenerative cortical spreading depression in anaesthetized cats. Read, et al. (1997) Cephalagia 17: 826.
Drug therapy is tailored to the severity and frequency of migraine headaches. For occasional attacks, acute treatment may be indicated, but for attacks occurring two or more times per month, or when attacks greatly impact the patient's daily life, prophylactic therapy may be indicated. The side effects of acute and prophylactic treatment agents including serotonin acting agents, beta-blockers, tricyclic antidepressants, anticonvulsants, and botulinum toxin type A injections can limit their use.
GABA modulates nociceptive input to the trigeminocervical complex mainly through GABAA receptors. Therefore GABAA receptors may provide a target for the development of new therapeutic agents for both acute and prophylactic treatment of headaches including migraines. Storer, et al. (2001) “GABA receptors modulate trigeminovascular nociceptive neurotransmission in the trigeminocervical complex.” Br J Pharmacol. 134(4): 896-904.
Nociceptive Pain
Nociceptive pain occurs in response to the activation of a specific subset of peripheral sensory neurons, the nociceptors. It is generally acute (with the exception of arthritic pain), self-limiting and serves a protective biological function by acting as a warning of on-going tissue damage. It is typically well localized and often has an aching or throbbing quality. Examples of include post-operative pain, sprains, bone fractures, burns, bumps, bruises, inflammation (from an infection or arthritic disorder), obstructions, and myofascial pain. Nociceptors are the nerves that sense and respond to parts of the body that suffer from damage. They signal tissue irritation, impending injury, or actual injury. When activated, they transmit pain signals (via the peripheral nerves as well as the spinal cord) to the brain. The pain is typically well localized, constant, and often with an aching or throbbing quality. Visceral pain is the subtype of nociceptive pain that involves the internal organs. It tends to be episodic and poorly localized.
Nociceptive pain is usually treated with opioids and/or non-steroidal anti-inflammatory drugs (NSAIDS) but due to low efficacy, unacceptable and even severe side effects, and addiction potential, their use can be limited. GABAA receptors are a target for therapeutics to treat nociceptive pain. Hara, et al. (2004) “The Interaction Between Gamma-Aminobutyric Acid Agonists and Diltiazem in Visceral Antinociception in Rats” Anesth Analg 98:1380-1384 reports that the combination of GABA agonists and L-type calcium channel blockers may be used to reduce visceral pain. However, GABAA agonists are known to have side effects, including sedation, dizziness, euphoria, nausea, and blurred vision. Therefore a great need exists for nociceptive pain therapeutics.
Neuropathic Pain
Neuropathic pain and nociceptive pain differ in their etiology, pathophysiology, diagnosis, and treatment. Neuropathic pain is a common type of chronic, non-malignant pain, which is the result of an injury or malfunction in the peripheral or central nervous system and serves no protective biological function. It is estimated to affect more than 1.6 million people in the U.S. population. Neuropathic pain has many different etiologies, and may occur, for example, due to trauma, diabetes, infection with herpes zoster (shingles), HIV/AIDS (peripheral neuropathies), late-stage cancer, amputation (including mastectomy), carpal tunnel syndrome, chronic alcohol use, exposure to radiation, and as an unintended side-effect of neurotoxic treatment agents, such as certain anti-HIV and chemotherapeutic drugs.
In contrast to nociceptive pain, neuropathic pain is frequently described as “burning,” “electric,” “tingling,” or “shooting” in nature. It is often characterized by chronic allodynia (pain resulting from a stimulus that does not ordinarily elicit a painful response, such as light touch) and hyperalgesia (increased sensitivity to a normally painful stimulus), and may persist for months or years beyond the apparent healing of any damaged tissues.
Taxol-induced peripheral neuropathy model (TIPN) is an art-accepted animal model of neuropathic pain. Cavaletti, et al. (May 1995) “Experimental peripheral neuropathy induced in adult rats by repeated intraperitoneal administration of taxol.” Exp Neurol. 133(1): 64-72. Injection of Taxol® intraperitoneally to female Wistar rats induced a peripheral neuropathy that resembles neuropathy in humans. Id. at pages 64; 69. An additional animal model of neuropathic pain comprises the single or five intraperitoneal administration of Taxol® (32 mg/kg) to male Sprague-Dawley rats. Authier, et al. (Dec. 29, 2000) “Description of a short-term Taxol-induced nociceptive neuropathy in rats.” Brain Res. 887(2): 239-49.
In a spinal cord injury (SCI) model of neuropathic pain, bumetanide, a NKCC1 antagonist, showed an analgesic effect suggesting that normal or elevated NKCC1 activity plays a role in the development and maintenance of SCI-induced neuropathic pain. Cramer, et al. (2008) “The role of cation-dependent chloride transporters in neuropathic pain following spinal cord injury.” Molecular Pain 4:36.
Neuropathic pain is difficult to treat. Analgesic drugs that are effective against nociceptive pain (e.g., opioid narcotics and non-steroidal anti-inflammatory drugs) are rarely effective against neuropathic pain. Similarly, drugs that have activity in neuropathic pain are not usually effective against nociceptive pain. The standard drugs that have been used to treat neuropathic pain appear to often act selectively to relieve certain symptoms but not others in a given patient (e.g., relief of allodynia, but not hyperalgesia). Bennett (1998) Hosp. Pract. (Off Ed). 33: 95-98. Treatment agents typically employed in the management of neuropathic pain include tricylic antidepressants (e.g., amitriptyline, imipramine, desimipramine, and clomipramine), systemic local anesthetics, and anti-epileptic drugs (AED) (e.g., phenytoin, carbamazepine, valproic acid, clonazepam, gabapentin, and pregabalin (LYRICA®)). See Lowther (September/October 2005) “Pharmacotherapy Update from the Department of Pharmacy” Vol. VIII, No. 5. Common side effects include over-sedation, dizziness, loss of memory and liver damage. Further, although traditionally not considered useful for the treatment of neuropathic pain, recent studies from genetically modified mice indicate that agents targeting only a subset of benzodiazepine (GABAA) receptors may provide pronounced antihyperalgesic activity against inflammatory and neuropathic pain. Zeilhofer, et al. (2009) “Subtype-selective GABAA receptor mimetics—novel antihyperalgesic agents?” J Mol Med 87:465-469. Therefore a great need exists for improved neuropathic pain therapeutics.
Neurotoxicity
A variety of chemical and biological agents, as well as some infectious agents, have neurotoxic effects. A common example is the pathophysiological effect of acute ethanol ingestion. Episodic ethanol intoxications and withdrawals, characteristic of binge alcoholism, result in brain damage. Animal models designed to mimic the effects of alcohol in the human have demonstrated that a single dose of ethanol given for 5-10 successive days results in neurodegeneration in the entorhinal cortex, dentate gyrus and olfactory bulbs, accompanied by cerebrocortical edema and electrolyte (Na+ and K+) accumulation. As with other neurodegenerative conditions, research has focused primarily on synaptically based excitotoxic events involving excessive glutamatergic activity, increased intracellular calcium and decreased γ-aminobutyric acid. A great need exists for improved neurotoxicity therapeutics.
Postherpetic Neuralgia
Postherpetic neuralgia (e.g., shingles, herpes zoster) is a painful condition affecting the nerve fibers and skin. Postherpetic neuralgia is a complication of shingles, a second outbreak of the varicella-zoster virus, which initially causes chickenpox. During an initial infection of chickenpox, some of the virus remains in the body, lying dormant inside nerve cells. Years later, the virus may reactivate, causing shingles. Once reactivated, the virus travels along nerve fibers causing pain. When the virus reaches the skin, it produces a rash and blisters. A case of shingles (herpes zoster) usually heals within a month. Some patients continue to feel pain long after the rash and blisters heal—a type of pain called postherpetic neuralgia. A variety of treatments for postherpetic neuralgia exist, although some do not experience complete relief from pain.
Postherpetic neuralgia results when nerve fibers are damaged during an outbreak of shingles. These damaged nerves suffer causing chronic, often excruciating pain that may persist for months—or even years—in the area where shingles first occurred. This complication of shingles occurs much more frequently in older adults. About 50 percent of adults older than 60 experience postherpetic neuralgia after shingles, whereas only 10 percent of all people with shingles do. The symptoms of postherpetic neuralgia are generally limited to the area of the skin where the shingles outbreak first occurred including sharp and jabbing, burning, or deep and aching pain; extreme sensitivity to touch and temperature change; itching and numbness; and headaches. In rare cases, patients might also experience muscle weakness or paralysis—if the nerves involved also control muscle movement. A great need exists for improved postherpetic neuralgia therapeutics.
Ocular Diseases (e.g., Vision Disorders, Ophthalmic Diseases)
It is estimated that the lifetime costs for all people with vision impairment who were born in 2000 will total $2.5 billion (2003 dollars). See generally, Centers for Disease Control and Prevention, Economic Costs Associated with Mental Retardation, Cerebral Palsy, Hearing Loss, & Vision Impairment, United States, 2003, MMWR 2004; 53:57-9. These costs include both direct and indirect costs. Direct medical costs, such as doctor visits, prescription drugs, and inpatient hospital stays, make up 6% of these costs. Direct nonmedical expenses, such as home modifications and special education, make up 16% of the costs. Indirect costs, which include the value of lost wages when a person dies early, cannot work, or is, limited in the amount or type of work he or she can do, make up 77% of the costs. These estimates do not include other expenses, such as hospital outpatient visits, emergency department visits, and family out-of-pocket expenses. The actual economic costs of vision impairment are, therefore, even higher than what is generally reported. U.S. Pat. No. 7,251,528.
Both NKCC and KCC2 are expressed in the outer and inner plexiform layers and colocalized in many-putative amacrine cells and in cells of the ganglion cell layer. However, the somata of putative horizontal cells displayed only NKCC immunoreactivity and many bipolar cells were only immunopositive for KCC2. In the outer retina, application of bumetanide, a specific inhibitor of NKCC activity, (1) increased the steady-state extracellular concentration of K+([K+](o)) and enhanced the light-induced decrease in the [K+](o), (2) increased the sPIII photoreceptor-dependent component of the ERG, and (3) reduced the extracellular space volume. In contrast, in the outer retina, application of furosemide, a specific inhibitor of KCC activity, decreased sPIII and the light-induced reduction in [K+](o), but had little effect on steady-state [K+](o). In the inner retina, bumetanide increased the sustained component of the light-induced increase in [K+](o). These findings thus indicate that NKCC and KCC2 control the [K+](o) and extracellular space volume in the retina in addition to regulating GABA- and glycine-mediated synaptic transmission. In addition, the anatomical and electrophysiological results together suggest that all of the major neuronal types in the retina are influenced by chloride cotransporter activity. Dmitriev, et al. (July-August 2007) “Multiple functions of cation-chloride cotransporters in the fish retina.” Vis Neurosci 24(4): 635-45.
The bumetanide-sensitive Na+K+2Cl− cotransporter (NKCC) also clearly contributes to the Cl− uptake into the pigmented epithelium (PE), This work reinforces the general consensus that active secretion of Cl−  is the major driving force of aqueous humor formation in mammalian eye and further substantiates the existence of species differences in the mechanism that accomplishes transepithelial Cl− transport. Kong, et al. (December 2006) “Chloride secretion by porcine ciliary epithelium: New insight into species similarities and differences in aqueous humor formation.” Invest Ophthalmol Vis Sci. 47(12): 5428-36.
Additionally, cation-chloride cotransporters are involved in retinal function by mediating neural computation in the retina. The directional responses of DS ganglion cells are mediated in part by the directional release of gamma-aminobutyric acid from starburst dendrites and that the asymmetric distribution of two cotransporters (K+Cl− cotransporter and Na+K+Cl− cotransporter) along starburst-cell dendrites mediates direction selectivity. Gavrikov, et al. (Dec. 23, 2003) “Cation-chloride cotransporters mediate neural computation in the retina.” Proc Natl Acad Sci USA 100(26): 16047-52;
Further, the function of retina depends on cation chloride transporters regulating GABA. In particular, different cation chloride cotransporters in retinal neurons allow for opposite responses to GABA. Thus, in the retina, the opposite effects of GABA on different cell types and on different cellular regions are probably primarily determined by the differential targeting of these two chloride transporters. See e.g., Barbour, et al. (May 1991) “Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina.” J Physiol. 436: 169-193; Keller, et al. (January 1988) “Regulation of intracellular pH in cultured bovine retinal pigment epithelial cells.” Pflugers Arch. 411(1): 47-52; and Vardi, et al. (Oct. 15, 2000) “Evidence that different cation chloride cotransporters in retinal neurons allow opposite responses to GABA.” Journal of Neuroscience 20(20): 7657-63. See also, Basu, et al. (November 1998) “Proton-driven dipeptide uptake in primary cultured rabbit conjunctival epithelial cells.” Invest Ophthalmol Vis Sci. 39(12): 2365-73; Cia, et al. (March 2005) Voltage-gated channels and calcium homeostasis in mammalian rod photoreceptors. J Neurophysiol. 93(3): 1468-75; Do, et al. (June 2006) “Swelling-activated Cl− channels support Cl− secretion by bovine ciliary epithelium.” Invest Ophthalmol Vis Sci. 47(6): 2576-82; Hunt, et al. (November 2005) “Aberrant retinal projections in congenitally deaf mice: how are phenotypic characteristics specified in development and evolution?” Anat Rec A Discov Mol Cell Evol Biol. 287(1): 1051-66; MacLeish and Nurse (July 2007) “Ion channel compartments in photoreceptors: evidence from salamander rods with intact- and ablated terminals.” J Neurophysiol. 98(1): 86-95; Mito, et al. (March 1993) “Calcium-dependent regulation of cation transport in cultured human nonpigmented ciliary epithelial cells.” Am J Physiol. 264(3 Pt 1): C519-26; Moody (1984) “Effects of intracellular H+ on the electrical properties of excitable cells.” Annu Rev Neurosci 7: 257-78; Mroz and Lechene (November 1993) “Extracellular N-methyl-D-glucamine leads to loss of hair-cell sodium, potassium, and chloride.” Hear Res. 70(2): 146-50; Schnetkamp (May 8, 1980) “Ion selectivity of the cation transport system of isolated intact cattle rod outer segments: evidence for a direct communication between the rod plasma membrane and the rod disk membranes. Biochim Biophys Acta. 598(1): 66-90; and Uhl and Desel (August 1989) “Optical probes of intradiskal processes in rod photoreceptors. II: Light-scattering study of ATP-dependent light reactions.” J Photochem Photobiol B. 3(4): 549-64.
Accordingly, a number of vision-threatening disorders of the eye presently do not have any effective therapies. One major problem in treatment of such diseases is the inability to deliver therapeutic agents into the eye and maintain them there at therapeutically effective concentrations. Therefore a great need exists for therapeutics to treat ocular diseases.
Parkinson's Disease
Parkinson's disease (PD) belongs to a group of conditions called motor system disorders, which result from the loss of dopamine-producing brain cells. The four primary symptoms of PD are tremor, or trembling in hands, arms, legs, jaw, and face; rigidity, or stiffness of the limbs and trunk; bradykinesia, or slowness of movement; and postural instability, or impaired balance and coordination. As these symptoms become more pronounced, patients may have difficulty walking, talking, or completing other simple tasks. PD usually affects people over the age of 50. Early symptoms of PD are subtle and occur gradually. Other symptoms may include depression and other emotional changes; difficulty in swallowing, chewing, and speaking; urinary problems or constipation; skin problems; and sleep disruptions. NINDS Parkinson's Disease Information Page (Sep. 23, 2009).
At present, there is no cure for PD, but a variety of medications provide dramatic relief from the symptoms. Usually, patients are given levodopa combined with carbidopa. Carbidopa delays the conversion of levodopa into dopamine until it reaches the brain. Nerve cells can use levodopa to make dopamine and replenish the brain's dwindling supply. Although levodopa-helps at least three-quarters of parkinsonian cases, not all symptoms respond equally to the drug. Bradykinesia and rigidity respond best, while tremor may be only marginally reduced. Problems with balance and other symptoms may not be alleviated at all. Anticholinergics may help control tremor and rigidity. Other drugs, such as bromocriptine, pramipexole, and ropinirole, mimic the role of dopamine in the brain, causing the neurons to react asthey would to dopamine. An antiviral drug; amantadine, also appears to reduce symptoms. In May 2006, the FDA approved rasagiline (AZILECT®) to be used along with levodopa for patients with advanced PD or as a single-drug treatment for early PD. NINDS Parkinson's Disease Information Page (Sep. 23, 2009).
Parkinson's disease (PD) is a neurodegenerative disorder that involves disruptions in GABA signaling. GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). PD destroys the input to striatal GABAergic neurons. Targeting GABA synthesis, degradation, transport, or receptors with new therapeutics may control GABA signaling, and therefore may be used for improved therapeutic treatments for Parkinson's disease. Kleppner and Tobin (2001) “GABA signaling: therapeutic targets for epilepsy, Parkinson's disease and Huntington's disease.” Expert Opin. Ther. Targets. 5(2):219-39.
Schizophrenia
Schizophrenia is a chronic, severe, and disabling brain disorder that affects about 1.1 percent of the U.S. population age 18 and older in a given year. People with schizophrenia sometimes hear voices others do not hear, believe that others are broadcasting their thoughts to the world, or become convinced that others are plotting to harm them. These experiences can make them fearful and withdrawn and cause difficulties when they try to have relationships with others. National Institute of Mental Health “Schizophrenia” website (2008).
Symptoms usually develop in men in their late teens or early twenties and women in the twenties and thirties, but in rare cases, can appear in childhood. They can include hallucinations, delusions, disordered thinking, movement disorders, flat affect, social withdrawal, and cognitive deficits. No cause of schizophrenia has been determined nor is there any curative therapy; however, antipsychotics are used in the treatment of symptoms. National Institute of Mental Health “Schizophrenia” website (2008).
Further, schizophrenia is associated with both decreased numbers and abnormalities in the distribution of GABAergic neurons in the cortex, particularly in the cortical laminae. Kaplan & Sadock's Comprehensive Textbook of Psychiatry (7th Ed) (2008). In the postmortem studies of schizophrenics, antipsychotic naive schizophrenics, and non schizophrenic controls, show a significant decrease in the number of GABA containing inter neurons, and a lessened amount of GABA production within these inter neurons in both of the schizophrenic groups. Nestler (1997) “Schizophrenia. An emerging pathophysiology. Nature 385(6617): 578-9. Therefore therapeutic agents that target the GABA system may be useful in treating schizophrenia.
Tinnitus
Tinnitus is the perception of sound within the human ear in the absence of corresponding external sound. Tinnitus is not a disease but a symptom resulting from a range of underlying causes that can include ear infections, foreign objects or wax in the ear, nose allergies that prevent (or induce) fluid drain and cause wax build-up, and injury from loud noises. Tinnitus can also be caused by hearing impairment and as a side-effect of some medications. Some cases of tinnitus are medically unexplained.
Tinnitus can be perceived in one or both ears or in the head. It is usually described as a ringing noise, but in some patients it takes the form of a high pitched whining, buzzing, hissing, screaming, humming, singing or whistling sound, or as ticking, clicking, roaring, “crickets” or “tree frogs” or “locusts,” tunes, songs, or beeping. It has also been described as a “whooshing” sound, as of wind or waves. Tinnitus can be intermittent or it can be continuous in which case it can be the cause of great distress. In some individuals, the intensity of tinnitus can be changed by shoulder, head, tongue, jaw, or eye movements. To date, no satisfactory therapeutics exists for tinnitus.
Partial deafferentation produces a loss of tonic inhibition in the auditory system that may lead to inappropriate neuroplastic changes eventually expressed as the pathophysiology of tinnitus. The pathological down-regulation of GABA provides a potential mechanism for this loss of inhibition. For example, in an animal model of tinnitus, vigabatrin, a GABA agonist, completely and reversibly eliminated the psychophysical evidence of tinnitus. Brozoski. et al. (2007) Vigabatrin, a GABA Transaminase Inhibitor, Reversibly Eliminates Tinnitus in an Animal Model. J Assoc Res Otolaryngol. 8(1): 105-118. Further, the disruption of the NKCC1 gene in mice causes hearing loss. Kahle, et al. (2004) Proc. Natl. Acad. Sci. USA 102(46): 16783-16788. Therefore, therapeutics targeting GABAergic system and/or NKCC1 may be useful in the treatment of tinnitus.
Withdrawal Syndrome
Withdrawal syndrome is generally associated with abnormal physical or psychological features that follow the abrupt discontinuation of a drug (e.g., medications, recreational drugs, and/or alcohol) that has the capability of producing physical dependence. (e.g., alcohol withdrawal syndrome, nicotine withdrawal syndrome, opioid withdrawal syndrome, benzodiazepine withdrawal syndrome, methadone withdrawal syndrome, SSRI discontinuation syndrome, hydrocodone withdrawal syndrome). Common withdrawal symptoms include sweating, tremor, vomiting, anxiety, insomnia, and muscle pain. There are different stages of withdrawal. Generally, a person will start to feel worse and worse, hit a plateau, and then the symptoms begin to dissipate. However, withdrawal from certain drugs (e.g., benzodiazepines, alcohol) can be fatal and therefore the abrupt discontinuation of any type of drug is not recommended.
Further, many additions involve compounds which affect the GABAerigic system including but not limited to alcohol and benzodiazepines. Therefore, when a person ceases use of the compound, the GABAergic system is involved in the symptoms of withdrawal syndrome. Nutt and Lingford-Hughes (2008) “Addiction: the clinical interface.” British Journal of Pharmacology 154(2): 397-405. Therefore agents that act on the GABAergic system may provide therapeutics to treat withdrawal syndromes.
Accordingly, there is a continuing need for compositions and methods for treatment and/or prophylaxis of diseases, disorders, and conditions that involve the Na+K+Cl− co-transporters (e.g., NKCC1 and NKCC2) including but not limited to addictive disorders, anxiety disorders, ascites, bipolar disorder, cancer, endothelial corneal dystrophy, edema, depression, epilepsy, glaucoma, ischemia, migraine, neuropathic pain, nociceptive neuralgia, ocular diseases, pain, postherpetic neuralgia, and schizophrenia. Additionally; there is a continuing need for compositions and methods for treatment and/or prophylaxis of diseases, disorders, and conditions that involve the GABAA receptors including but not limited to Alzheimer's Disease, addictive disorders, anxiety disorders, autism, bipolar disorder, depression, epilepsy, Huntington's Disease, insomnia, migraine, neuropathic pain, nociceptive pain, pain, Parkinson's disease, personality disorders, psychosis, schizophrenia, seizure disorders, tinnitus, and withdrawal syndromes.